Can't help with the raincover thing, but.... pinholes can be repaired with a product called "liquid electrical tape. It works best if you thin it first, and give an offending pinhole a couple of coats. The area of application doesn't have to be much bigger than the pinhole. Use a little modeling paint brush from a hobby store. Do it (hehe I said "Do it" hehehe hehe) on the inside of the bellows, and nobody will know. Don't compress the bellows until it is totally dry, for if you do, your camera will have a new, shorter, max bellows extension. Make sure you get the black version, I found mine at the local big box HI. I used this stuff to fix pinholes in the curtain shutter of my Exakta, and it worked like a champ. The shutter of that camera sees allot more stress than a bellows. Prudence says test it on one pinhole first, to make sure it behaves with your particular material.
Oh, welcome to APUG Hugh, if I didn't say so somewhere else.